This invention relates to a method of handling an anchoring or mooring line comprising a rope, a chain, and a coupling link connecting the rope and the chain, in the process of hauling the line in.
An important use of a combined line of the type described is for the anchoring or mooring of offshore drilling rigs or vessels, particularly semi-submersible drilling rigs operating at moderate or great sea depths.
Though the advantages of this type of mooring line are generally recognized by experts -- the chain part of the line permitting vertical movement of the drilling rig or vessel under the influence of even high waves without over-stressing of the line, the rope part limiting horizontal displacement -- it is still more usual to employ either a chain alone or a rope alone. The reason for this is the difficulties encountered in passing the coupling link through the required bends of the path in which the line must be guided onboard the rig, and in changing over from hauling in rope by means of a winch drum or the like to hauling in chain by means of a wildcat or the like. It has been attempted to overcome these difficulties by stowing the chain on a service vessel from which the chain is connected to the rope outside the drilling rig or main vessel when the rope is paid out from the latter, but this method is time consuming, expensive and dangerous.
The dismounting of the connection between the wire rope and the chain may also take place onboard. The line with the coupling link must then first pass around a fairlead, and when the upper end of the chain has arrived at a sutiable height, the haul-in is stopped. The chain is now locked by means of a chain stop or locking lever, whereafter the winch drum on which the rope is wound can be released, and the coupling link can be disconnected. The chain must then be connected to a length of chain, which is preferably fastened within the chain box, before haul-in can be continued, now by means of the wildcat.
While these operations may be performed without difficulty in nice weather, when the chain is new and the chain load is moderate, serious problems may arise at low temperatures, in heavy weather, if the coupling link has been subjected to deformation or corrosion, or if the load on the chain is heavy.